The tracking microscope is being used to characterize photo responses due to rhodopsin and to at least one other pigment in the bacterium Halobacterium halobium. Mutants are being isolated and characterized that may have altered rhodopsins, altered receptor membranes, or defects involving the coupling of the receptors to the flagella. Analysis of the mutants is expected to reveal the sequence of events leading from the excitation of the rhodopsin to the control of the flagella, and to an understanding of their molecular bases.